Portable two-way radios are becoming more advanced in both form and function. Today, portable two-way radios are being used to communicate with extremely complicated communication systems which automatically switch operating frequencies and signalling protocols. These systems typically provide display message capabilities as well as automatic interface to land-based telephone interconnect systems. One such system is manufactured by Motorola, Inc. and is referred to as a "Trunked Radio System". Trunked Radio systems utilize a central control unit to organize and control the operation of the entire system. Users of the system communicate with the central control unit over a dedicated data channel to request and receive channel access information. During the course of a long conversation, the user may actually switch between several frequencies, as determined by the central control unit. The central control unit also couples to land-line telephone interconnect service, to provide users of the Trunked Radio System access to telephone service via radio. Various aspects and features of trunked radio systems are described in U.S. Pat. No. 4,012,597, Mar. 15, 1977 entitled "Transmission Trunk Multi-Channel Dispatch System with priority Queuing", by Lynk et al., The operation of Trunked radio systems is further described in a Motorola instruction manual no. R4-184B, entitled "System Planner; Trunked Radio Systems", July 1980, and Motorola instruction manual entitled "Trunked System Central Controller", numbers 68P81047E50 and 68P81066E60, available from the Service publications Department of Motorola, Inc., 1301 E. Algonquin Road, Schaumburg, Ill. Trunked Radio Systems interface to conventional telephone interconnect systems through a telephone interconnect control unit. The operation of a telephone interconnect control unit is described in Motorola instruction manual 68P81063E20 entitled "Trunked System Central Controller", available from the address above, said manuals and Patent herein incorporated by reference. Other aspects of Trunked Radio Systems which interface to telephone interconnect service are described in U.S. patent application Ser. No. 637,515, entitled "Dynamic Control of Telephone Traffic on a Trunked Radio System", by Zdunek et al. filed Aug. 8, 1984.
The user units of a trunked radio system must necessarily be configured with a keypad entry system and a display unit. The display unit is used to provide important system and unit status to the user. The keypad entry system provides a means by which alphanumeric and telephone dialing information can be exchanged with the central control unit. In trunking, the keypad entry system serves for quick entry of subfleet selections, which is an alphabetic character from A-P. It also enters alphabetic characters to the display to send them as text data or messages over the control channel or a separate voice channel. In conventional systems, text data could similarly be entered and displayed at the base station. The text data, names, or messages may also be decoded at the base station and provide a command to execute further functions; i.e., a page sequence, a telephone number, or an alarm.
A standard approach to entering alphanumeric characters on a display uses individual keys for each character. This approach has the disadvantage that it requires many keys, and therefore, a large area to be implemented. In portable two-way units, "real-estate" is at a premium, and standard telephone-type keypads are used. Typically, each numeric value for the telephone-type keypad corresponds to three alphanumeric symbols. For example, the numeral 2 corresponds to the letters A, B, and C, etc. All keypad entry systems which use the telephone type keypad are faced with several problems. First, since each key represents more than one character, some means of decoding each key must be provided. However, the letters Q and Z are not represented on the keypad. In addition, each time a key is depressed, some information is entered. Therefore, some means of preventing erroneous information from being entered is essential. Prior keypad entry systems use mechanical switches to disable the keypad when not in use. This requires another switch to be added to the radio controls. Previous designs have not taken advantage of the telephone keypad structure because of the difficulty of retaining electronic information after the radio has been turned off.